Compression ignition internal combustion engines contain one or more reciprocating pistons located within respective combustion chambers of an engine block. Associated with each piston is a fuel injector that sprays a highly pressurized fuel into the combustion chamber. The fuel is mixed with air that is introduced into the combustion chamber through one or more intake valves. After combustion, the exhaust gas flows out of the combustion chamber through one or more exhaust valves. The injection of fuel and movement of the intake and exhaust valves are typically controlled by mechanical cams. Valve cams are relatively inefficient and susceptible to wear. Additionally, the cams do not allow the engine to vary the timing of fuel injection, or the opening and closing of the intake/exhaust valves independent of engine speed.
U.S. Pat. No. 5,255,641 issued to Schechter and assigned to Ford Motor Co. and U.S. Pat. No. 5,339,777 issued to Cannon and assigned to Caterpillar Inc. disclose hydraulically driven intake/exhaust valves that do not require cams to open and close the valves. The movement of the intake/exhaust valves are controlled by a solenoid actuated fluid valve(s). When the fluid valve(s) is in one position, a hydraulic fluid flows into an enclosed stem portion of the intake/exhaust valve. The hydraulic fluid exerts a force on the stem which opens the valve. When the fluid valve(s) is switched to another position, the intake/exhaust valve moves back to its original closed position. The fluid valve(s) is switched by an electronic controller. The controller can vary the timing of the intake/exhaust valves to optimize the performance of the engine.
The solenoid actuated fluid valves are typically connected to a single microprocessor which can vary the valve timing in response to variations in a number of input parameters such as fuel intake, hydraulic rail pressure, ambient temperature, etc. The microprocessor can vary the start time and the duration of the driving signal provided to the fluid valves to obtain a desired result. Because of variations in manufacturing tolerances, different valves may have different responses to the same driving pulse. For example, given the same driving pulse, one intake valve may open for a shorter period of time than another intake valve in the same engine.
The Schechter patent discusses a process wherein each valve is calibrated to determine a correction value. The correction value is stored within the electronics of the engine and used to either shorten or lengthen the driving pulse provided to each valve so that each of the valves are open for the same time duration. Although effective in compensating for variations in manufacturing tolerances, the Schechter technique does not compensate for variations that occur during the life of the engine. For example, one of the valves may begin to stick and require more energy to move into an open position.
The camless intake valve(s) is typically actuated by a dedicated control valve which can either open or close the valve. The intake valve orifice area is the same each time the intake valve(s) is open. Likewise, the exhaust valve(s) may be controlled by a dedicated control valve such that the valve orifice area is the same each time the valve(s) is open. It may be desirable to vary the orifice area and the corresponding flow of air and exhaust gases to and from the combustion chamber. Such a configuration would provide another variable that can be used by the microcontroller to optimize the fuel consumption, power, emissions, etc. of the engine.
Some internal combustion engines contain a "turbocharger" which pushes air into the combustion chambers. Turbochargers are typically driven by the flow of exhaust gases from the combustion chamber. The pressures within a combustion chamber are very high particularly at a piston top dead center position. Opening the exhaust valves at such high pressures typically requires a large amount of work. Consequently, the exhaust valves are typically not opened until the piston has moved toward a bottom dead center position. At this position, the exhaust gas pressure is relatively low. The low exhaust gas pressure may not be as effective in driving the turbocharger as a higher exhaust gas pressure. It would be desirable to provide a valve assembly which would allow the exhaust valves to be opened at any time during a cycle of an engine.